Kinetic study of CH3 + HBr and CH3 + Br reactions by laser photolysis-transient absorption over 1-100 bar pressure range

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Abstract

Reactions of methyl radicals with hydrogen bromide CH3 + HBr → CH4 + Br (1) and bromine atoms CH3 + Br → CH3Br (2) were studied using excimer laser photolysis-transient UV spectroscopy at 297 ± 3 K over the 1-100 bar buffer gas (He) pressure range. Methyl radicals were produced by 193 nm (ArF) laser photolysis of acetone, (CHa3)2CO, and methyl bromide, CH3Br. Temporal profiles of methyl radicals were monitored by UV absorption at 216.51 nm (copper hollow cathode lamp with current boosting). The yield of acetyl radicals in photolysis of acetone at 193 nm was found to be less than 5% at 100 bar He based on the transient absorptions at 222.57 and 224.42 nm. The measured rate constants for reaction 1 are k1 = (2.9 ± 0.7) × 10-12, (3.8 ± 1.5) × 10-12, and (3.4 ± 1.3) × 10-12 cm3 molecule-1 s-1 at the buffer gas (He) pressures of 1.05, 11.2, and 101 bar, respectively. The rate data obtained in this study confirmed high values of the previous (low pressure) measurements and ruled out the possibility of interference of excited species. The measured rate constant is independent of pressure within the experimental error. The rate constant of reaction of methyl radicals with bromine atoms (2) was determined relative to the rate constant of methyl radical self-reaction, CH3 + CH3 → C2H6 (3) in experiments with photolysis of CH3Br: k2/k3 = 0.92 ± 0.32, 1.15 ± 0.30, and 1.65 ± 0.26 at 1.05, 11.2, and 101 bar He, respectively. On the basis of the literature data for reaction 3, this yields k2 = (5.8 ±2.2) × 10-11, (7.4 ± 2.2) × 10-11, (10.7 ± 2.3) × 10-11, and (11.9 ± 2.5) × 10-11 cm3 molecule-1 s-1 at 1.05, 11.2, 101 bar (He), and in the high-pressure limit, respectively.

Original languageEnglish (US)
Pages (from-to)7958-7971
Number of pages14
JournalJournal of Physical Chemistry A
Volume103
Issue number40
DOIs
StatePublished - Oct 7 1999

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry

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